In the movie based on the chilling true story of Bundy's crimes, the actress transformed into the warm-hearted girlfriend Liz who is unaware of her boyfriend Ted's murder spree during the 1970s in the US.
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The Love, Rosie star, 30, explained it was 'helpful' to have the opportunity to meet the 'gracious' woman she plays during her appearance on Tuesday's This Morning.
She told Holly Willoughby and Philip Schofield: 'I went and met the woman that I play. It was really helpful. She was so gracious, giving me material to look at and speaking to me.'
The To The Bone star gushed Liz was a 'positive light' on the movie set of the thriller where Lily filmed scenes with her co-star Zac Efron.
She said: 'I don't know if she's going to see the movie because it's difficult but within the filming process she came on set and she was a positive light on set. You wouldn't expect that with what happened. She gave us her support. She's really lovely.'
Alongside Lily, Zac Efron gives a convincing portrayal of the charming yet troubled serial killer Ted who terrorised the US with his violent murders.
The thriller opens with the picture-perfect early days of their romance before the serial killer embarks on a barbaric murder spree.
Bundy was executed in Florida in 1989 after he confessed to murdering 36 women during his reign of terror.
Liz became suspicious of Bundy five years into their relationship and tried to turn him into the police.
She recorded her experiences in memoir The Phantom Prince: My Life with Ted Bundy which was published when she was 36 years old and Bundy was on death row.
The couple first crossed paths in 1969 and they dated until they parted ways for good in 1980.
As well as meeting Ted's girlfriend, Lily also insisted she wouldn't look at any crime scene imagery Elizabeth wouldn't have seen to ensure she mastered the part.
She said: 'I play the girlfriend, she had a daughter, and they were a family unit before he committed the crimes. In preparation for the film, I wouldn't look at any imagery. I didn't want to look at anything Liz hadn't seen.'
In another interview, Zac said he wanted to make the movie for Bundy's victims while he insisted the film didn't glamorise the serial killer's crimes.
He told The Guardian: 'Not glamorous or standing for a cause. Ted's dead. And karma... I wanted to make this film for the victims.
'Ted Bundy himself said that murderers don't come crawling out of the dark.' It's been a busy time for Lily as she has also taken the leading lady role of Edith Bratt in the Tolkien biopic which retells the story of the author's real love story.
Orphans Edith and Tolkien fell in love when they crossed paths for the first time at the home where they both lived. She said: 'It was true. It gives us all hope that [love] exists.'
The actress confessed she did originally audition to be in the 2012 movie The Hobbit as an Elvin character but she didn't get the part.
In a twist of fate, she ended up playing the woman who inspired the very same character just seven years later.
She said: 'I did audition for the Elvin character for the hobbit, I didn't get it. I became the woman who inspired them.'
Tolkien's great grandson Callum joined Lily and her on-screen lover Nicholas Hoult, who plays the man himself, in the battle sequences.
